The invention relates to a crane having a hydraulically actuated jib system, at least one cylinder for actuating the jib system and an apparatus for monitoring the pressure in the cylinder, which indicates the crane loading, wherein the feed flow of hydraulic fluid to the cylinder is controllable by varying the cross-section of a valve.
When moving loads, the acceleration effects which occur can cause oscillations of the carrier structure of the crane or the load carried by the crane. For example, the situation may involve oscillations in a vertical direction when a load is moved in a vertical direction, that is to say when it is raised or lowered, and that movement is stopped. Such oscillations give rise to loading peaks in the carrier structure of the crane and/or uncontrolled movements of the load.
The conventional method of actively damping undesired oscillations is a regulation effect, wherein the oscillation is damped by an oscillation of the same frequency, which is phase-shifted through 180.degree.. The method is particularly suitable for preventing the progressive build-up of oscillaticns, for example pendulum movements of loads which are being moved, in the case of portal cranes (see DE 42 38 795 A1). The time involved in determining the phase and frequency of the harmonic oscillation causing the problems is however relatively great. In particular loadings which suddenly occur are not alleviated precisely in their dangerous initial phase.
In comparison, for example DE 37 19 897 A1 has proposed that the dynamic overloadings which most frequently occur in the case of loading cranes for goods vehicles or trucks can be combated in terms of the cause thereof. In that respect, the cause is considered to be excessively fast displacement of the control lever for the hydraulic drive of a crane member. The great difference between the reference and actual values of the speed of the crane member or the reference and actual values of the valve cross-section, Without appropriate countermeasures, would cause an excessively fast change in the valve cross-section and thus excessively great acceleration of the crane. In accordance with the known proposal therefore the control signal is filtered, and the information about the position of the control lever and thus the desired final speed of the crane member is only transmitted with a delay to the control motor for varying the valve cross-section. A disadvantage with that method is in particular that it only corrects operating errors but it does not take account of the actual dynamic conditions These however are dependent not only on the control but on the respective configuration of the crane and the load as well as environmental influences (wind, or a sudden reduction or increase in load).